The wmb() in next_desc seems out of place and the comments are inaccurate. Remove the unnecessary barrier and clean up next_desc(). next_desc() is called by virt_queue__get_head_iov() when filling the iov with desciptor addresses. It reads the descriptor's flag and next index. The virt_queue__get_head_iov() only reads the direct and indirect descriptors, and doesn't write any shared memory except from iov and cursors that will be read by the caller. As far as I can see, vhost (the kernel implementation of virtio device) does well without any barrier here, so I think it might be safe to remove. Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx> ---- virtio/core.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/virtio/core.c b/virtio/core.c index ddce48bf6ab6..0e2646c61b91 100644 --- a/virtio/core.c +++ b/virtio/core.c @@ -75,8 +75,8 @@ static inline bool virt_desc__test_flag(struct virt_queue *vq, /* * Each buffer in the virtqueues is actually a chain of descriptors. This - * function returns the next descriptor in the chain, or vq->vring.num if we're - * at the end. + * function returns the next descriptor in the chain, or max if we're at the + * end. */ static unsigned next_desc(struct virt_queue *vq, struct vring_desc *desc, unsigned int i, unsigned int max) @@ -87,12 +87,10 @@ static unsigned next_desc(struct virt_queue *vq, struct vring_desc *desc, if (!virt_desc__test_flag(vq, &desc[i], VRING_DESC_F_NEXT)) return max; - /* Check they're not leading us off end of descriptors. */ next = virtio_guest_to_host_u16(vq, desc[i].next); - /* Make sure compiler knows to grab that: we don't want it changing! */ - wmb(); - return next; + /* Ensure they're not leading us off end of descriptors. */ + return min(next, max); } u16 virt_queue__get_head_iov(struct virt_queue *vq, struct iovec iov[], u16 *out, u16 *in, u16 head, struct kvm *kvm) -- 2.16.2
